Quantum Monte Carlo Studies of Superfluid Fermi Gases

We report results of quantum Monte Carlo calculations of the ground state of dilute Fermi gases with attractive short range two-body interactions. The strength of the interaction is varied to study different pairing regimes which are characterized by the product of the s-wave scattering length and the Fermi wave vector, $ak_F$. We report results for the ground state energy, the pairing gap $\Delta$ and the quasiparticle spectrum. In the weak coupling regime, $1/ak_F < -1$, we obtain BCS superfluid and the energy gap $\Delta$ is much smaller than the Fermi gas energy $E_{FG}$. When $a > 0$, the interaction is strong enough to form bound molecules with energy $E_{mol}$. For $1/ak_F \gtrsim 0.5$ we find that weakly interacting composite bosons are formed in the superfluid gas with $\Delta$ and gas energy per particle approaching $|E_{mol}|/2$. In this region we seem to have Bose-Einstein condensation (BEC) of molecules. The behavior of the energy and the gap in the BCS to BEC transition region, $-0.5 < 1/ak_F < 0.5$ is discussed.